function [newpop]=crossover(oldpop,stringlength,a,b,pc);
pops=size(oldpop,1);
newpop=zeros(size(oldpop));
for i=1:pops/2
    if(rand<pc)
          cpoint=round(rand*(stringlength-2))+1;
          newpop(i,:)=[oldpop(i,1:cpoint),oldpop(pops/2+i,cpoint+1:stringlength),0,0];
          newpop(pops/2+i,:)=[oldpop(pops/2+i,1:cpoint),oldpop(i,cpoint+1:stringlength),0,0];         
    else
          newpop(i,:)=oldpop(i,:);
          newpop(pops/2+i,:)=oldpop(pops/2+i,:);
    end
end
     sum=0;
     for j=1:stringlength
         sum=sum+newpop(:,j)*2^(j-1);  
         newpop(:,stringlength+1)=sum;%解碼
         x=newpop(:,stringlength+1);
         newpop(:,stringlength+2)=fitness(x,pops,stringlength)
     end